Fischer-tropsch synthesis process

ABSTRACT

A process for the conversion of synthesis gas to higher hydrocarbons by synthesis gas, at an elevated temperature and pressure, with a suspension of a particulate Fischer-Tropsch catalyst, in a system comprising at least one high shear mixing zone and a reactor vessel wherein the process comprises: (a) passing the suspension and the gaseous stream through the high shear mixing zone(s) wherein the gaseous stream is broken down into gas bubbles and/or irregularly shaped gas voids; (b) discharging suspension having gas bubbles and/or irregularly shaped gas voids dispersed therein from the high shear mixing zone(s) into the reactor vessel; and (c) maintaining the temperature of the suspension discharged into the reactor vessel at the desired reaction temperature by means of an internal heat exchanger positioned within the suspension in the reactor vessel characterized in that at least 5% of the exothermic heat of reaction is removed from the system by means of the internal heat exchanger. The remainder of the exothermic heat of reaction may be removed from the system by means of an external heat exchanger and/or through the introduction of a liquid coolant.

[0001] This application is the U.S. National Phase of InternationalApplication PCT/GB02/02307, filed May 17, 2002, which designated theU.S.

[0002] The present invention relates to a process for the conversion ofcarbon monoxide and hydrogen (synthesis gas) to liquid hydrocarbonproducts in the presence of a Fischer-Tropsch catalyst.

[0003] In the Fischer-Tropsch synthesis reaction a gaseous mixture ofcarbon monoxide and hydrogen is reacted in the presence of a catalyst togive a hydrocarbon mixture having a relatively broad molecular weightdistribution. This product is predominantly straight chain, saturatedhydrocarbons which typically have a chain length of more than 2 carbonatoms, for example, more than 5 carbon atoms. The reaction is highlyexothermic and therefore heat removal is one of the primary constraintsof all Fischer-Tropsch processes. This has directed commercial processesaway from fixed bed operation to slurry systems. Such slurry systemsemploy a suspension of catalyst particles in a liquid medium therebyallowing both the gross temperature control and the local temperaturecontrol (in the vicinity of individual catalyst particles) to besignificantly improved compared with fixed bed operation.

[0004] Fischer-Tropsch processes are known which employ slurry bubblecolumns in which the catalyst is primarily distributed and suspended inthe slurry by the energy imparted from the synthesis gas rising from thegas distribution means at the bottom of the slurry bubble column asdescribed in, for example, U.S. Pat. No. 5,252,613.

[0005] The Fischer-Tropsch process may also be operated by passing astream of the liquid medium through a catalyst bed to support anddisperse the catalyst, as described in U.S. Pat. No. 5,776,988. In thisapproach the catalyst is more uniformly dispersed throughout the liquidmedium allowing improvements in the operability and productivity of theprocess to be obtained.

[0006] GB 728543 relates to a process for the production of hydrocarbonsby the reaction of synthesis gas in the presence of a catalyst which maybe suspended in finely divided form within the hydrocarbon oil (contactoil). A mechanically moved stream of contact oil circulating after theseparation of the gas, and the synthesis gas is introduced into thereaction chamber below a cooling arrangement disposed therein, suitablyby means of one or a series of nozzles. Cooling of the contact oil ormixture of contact oil and gas in the reaction chamber is effected in anumber of stages in such manner that the mixture of synthesis gas andcontact oil successively flows through cooling stages at increasingtemperature. Owing to the fact that the individual cooling stages have atemperature increasing from the bottom upwards, the reaction can beretarded in places where the concentration of carbon monoxide andhydrogen is highest, namely in the lower part of the reaction tower, bythe application of low temperatures. In accordance with the reduction ofthe concentration of the reaction substances, the temperature is thenincreased in the higher zones of the reaction tower, so that thecomplete reaction between the carbon monoxide and the hydrogen,corresponding substantially to equilibrium, is obtained in theneighborhood of the top of the reaction tower. Thus, GB 728,543 relatesto a plug flow reactor vessel where the reaction conditions vary in theindividual cooling stages.

[0007] We have recently found that a Fischer-Tropsch process may beoperated by contacting synthesis gas with a suspension of catalyst in aliquid medium in a system comprising at least one high shear mixing zoneand a reactor vessel. The suspension is passed through the high shearmixing zone(s) where synthesis gas is mixed with the suspension underconditions of high shear. The shearing forces exerted on the suspensionin the high shear mixing zone(s) are sufficiently high that thesynthesis gas is broken down into gas bubbles and/or irregularly shapedgas voids. Suspension having gas bubbles and/or irregularly shaped gasvoids dispersed therein is discharged from the high shear mixing zone(s)into the reactor vessel where mixing is aided through the action of thegas bubbles and/or the irregularly shaped gas voids on the suspension.The suspension present in the reactor vessel is under such highlyturbulent motion that any irregularly shaped gas voids are constantlycoalescing and fragmenting on a rapid time scale, for example over atime frame of up to 500 milliseconds, typically between 10 and 500milliseconds. The transient nature of these irregularly shaped gas voidsresults in improved heat transfer and mass transfer of gas into theliquid phase of the suspension when compared with a conventional slurrybubble column reactor. The reactor system may therefore be regarded as acontinuous stirred tank reactor (CSTR) with the conditions oftemperature and pressure and the concentration of reactants and productsbeing substantially constant throughout the body of suspension in thereactor vessel. The reactor vessel may be a tank reactor in which case asuspension recycle stream is withdrawn from the reactor vessel and maybe recycled to the high shear mixing zone(s) via an external conduit.Exothermic heat of reaction may be removed from the system by means of aheat exchanger positioned in the external conduit (external heatexchanger). Optionally, further exothermic heat of reaction may beremoved from the system by means of a heat exchanger, for example,cooling tubes or coils positioned within the suspension in the reactorvessel (internal heat exchanger). This process is described in WO0138269 (PCT patent application number GB 0004444) which is hereinincorporated by reference. However, a problem may arise when operatingthe process of WO 0138269 in that there maybe a limit on the temperatureto which the suspension may be cooled by the external heat exchangerowing to the risk of quenching the reaction and/or deactivating thecatalyst. In the absence of an internal heat exchanger, this maynecessitate circulating suspension around the external loop conduit atan uneconomic flow rate.

[0008] It has now been found that where a slurry process is operated ina reactor system comprising at least one high shear mixing zone and areactor vessel that it is advantageous to remove at least a 5% of theexothermic heat of reaction from the system by means of an internal heatexchanger.

[0009] Accordingly, the present invention relates to a process for theconversion of synthesis gas to higher hydrocarbons by contacting agaseous stream comprising synthesis gas, at an elevated temperature andpressure, with a suspension comprising a particulate Fischer-Tropschcatalyst suspended in a liquid medium, in a system comprising at leastone high shear mixing zone and a reactor vessel wherein the processcomprises:

[0010] (a) passing the suspension and the gaseous stream through thehigh shear mixing zone(s) wherein the gaseous stream is broken down intogas bubbles and/or irregularly shaped gas voids;

[0011] (b) discharging suspension having gas bubbles and/or irregularlyshaped gas voids dispersed therein from the high shear mixing zone(s)into the reactor vessel;

[0012] (c) maintaining the temperature of the suspension discharged intothe reactor vessel at the desired reaction temperature by means of aninternal heat exchanger positioned within the suspension in the reactorvessel characterized in that at least 5% of the exothermic heat ofreaction is removed from the system by means of the internal heatexchanger.

[0013] For avoidance of doubt, conversion of synthesis gas to higherhydrocarbons may be initiated in the high shear mixing zone(s). However,it is envisaged that the majority of the conversion of the synthesis gasto higher hydrocarbons will take place in the reactor vessel.

[0014] Typically, at least 10%, preferably at least 20%, more preferablyat least 40%, most preferably at least 50% of the exothermic heat ofreaction is removed from the system by means of the internal heatexchanger. It is envisaged that substantially all of the exothermic heatof reaction may be removed from the system by means of the internal heatexchanger. However, it is preferred that between 20 to 50%, morepreferably 30 to 50% of the exothermic heat of reaction is removed fromthe system by means of the internal heat exchanger.

[0015] Preferably, suspension is withdrawn from the reactor vessel andis at least in part recycled to the high shear mixing zone(s)(hereinafter referred to as “suspension recycle stream”). Suitably, thesuspension recycle stream is cooled outside of the reactor vessel andhigh shear mixing zone(s) by means of a further heat exchanger(hereinafter “external heat exchanger”) in order to further assist inthe removal of exothermic heat of reaction from the system. Preferably,between 20 to 55%, more preferably, 30 to 50%, for example 40 to 50% ofthe exothermic heat of reaction is removed from the system in theexternal heat exchanger. Preferably, the suspension recycle stream iscooled to a temperature not more than 30° C. below, preferably, not more12° C. below, the temperature of the suspension in the reactor vessel.

[0016] Preferably, suspension is withdrawn from the reactor vessel andis recycled to the high shear mixing zone(s) by means of an externalconduit having a first end in communication with an outlet (for thesuspension) in the reactor vessel and a second end in communication withthe high shear mixing zone(s). Suitably, the external heat exchanger maybe positioned on the external conduit. Preferably, the ratio of thevolume of the external conduit (excluding the volume of the externalheat exchanger) to the volume of the reactor vessel is in the range of0.005:1 to 0.2:1. Suitably, a mechanical pumping means, for example, aslurry pump is positioned in the external conduit, preferably upstreamof the external heat exchanger.

[0017] For a 30,000 barrel/day commercial scale plant, the suspension issuitably recycled through the external conduit at a rate of between10,000 to 50,000 m³/h, preferably, 15,000 to 30,000 m³/h, morepreferably 17,000 to 25,000 m³/h. For a 60,000 barrel/day commercialplant, the suspension is recycled through the external conduit at a rateof between 20,000 to 100,000 m³/h. Thus, the rate at which thesuspension is recycled through the external conduit will be pro rata tothe size of the commercial scale plant. Suitably, the flow rate throughthe external conduit may be in the range (n×10,000) m³/h to (n×50,000)m³/h for a (n×30,000) barrel/day commercial plant where n is a number inthe range 0.25 to 10, preferably 1.5 to 5.

[0018] Preferably, up to 50% by volume, more preferably up to 20% byvolume of the hydrogen component of the synthesis gas (hereinafter“hydrogen component”) is introduced into the suspension recycle stream.

[0019] Without wishing to be bound by any theory, it is believed thatthe unconverted synthesis gas which is present in the suspension recyclestream may be depleted in hydrogen. An advantage of injecting thehydrogen component into the suspension recycle stream is that this willmaintain the ratio of hydrogen to carbon monoxide in the synthesis gasat an optimum value thereby improving the conversion of synthesis gas tohigher hydrocarbons. A further advantage of injecting the hydrogencomponent into the suspension recycle stream is that this may alsostabilize the catalyst.

[0020] It is also envisaged that up to 50% by volume, preferably up to20% by volume of the carbon monoxide component of the synthesis gas(hereinafter “carbon monoxide component”) may be introduced into thesuspension recycle stream.

[0021] Suitably, the hydrogen component and/or the carbon monoxidecomponent is introduced into the external conduit either upstream ordownstream of the mechanical pumping means, preferably downstream of themechanical pumping means. Preferably, the hydrogen component and/orcarbon monoxide component is introduced to the external conduit upstreamof the external heat exchanger. The hydrogen component and/or the carbonmonoxide component may be introduced into the external conduit at morethan one position along the length of the external conduit.

[0022] Preferably, the hydrogen component and/or the carbon monoxidecomponent is introduced into the external conduit via a gas nozzle.Preferably, the pressure drop over the gas nozzle is at least 0.1 bar,more preferably, at least 0.5 bar, for example, at least 1 bar.

[0023] Where necessary, the ratio of hydrogen to carbon monoxide in theunconverted synthesis gas within the reactor vessel may be adjusted byfeeding additional hydrogen and/or carbon monoxide directly into thereactor vessel, for example, via a gas sparger.

[0024] Where the hydrogen component is introduced into the suspensionrecycle stream in the substantial absence of carbon monoxide, thehydrogen may be obtained from synthesis gas, for example, the hydrogenmay be separated from synthesis gas by pressure swing adsorption or bydiffusion through a membrane system.

[0025] Suitably, the reactor vessel has a diameter of from 5 to 15metres, preferably 7.5 to 10 metres, more preferably 7.5 to 8 metres.Suitably, the reactor vessel has a length of from 5 to 30 metres,preferably 10 to 20 metres, for example, 15 to 20 metres. For practicalreasons, the reactor vessel may be operated with a headspace. Where thereactor vessel has a headspace, the height of the suspension in thereactor vessel is preferably at least 7.5 metres, preferably at least 10metres.

[0026] Preferably, the reactor vessel approximates to a continuousstirred tank reactor (CSTR) having a Peclet number of less than 3, morepreferably less than 1, even more preferably approaching zero. ThePeclet (Pe) number is defined by the equation:

Pe=U _(g) H/δ

[0027] where U_(g) is the gas velocity (ms⁻¹), H is the height of thesuspension in the reactor vessel (m), δ is the dispersion coefficient(m²s⁻¹) (see Carberry, J. J., ‘Chemical and Catalytic ReactionEngineering’, McGraw-Hill, 1976, or Levenspiel, O., ‘Chemical ReactionEngineering’, Wiley, 1972.).

[0028] Owing to the well mixed nature of the suspension in the reactorvessel, it is possible to operate the internal heat exchanger with alarge temperature difference between the coolant liquid which is fed tothe heat exchanger (hereinafter referred to as “heat exchange liquid”)and the temperature of the suspension in the reactor vessel, without anyrisk of quenching the Fischer-Tropsch synthesis reaction. Thus, the heatexchange liquid fed to the internal heat exchanger is preferably at atemperature which is at least 12° C. below, more preferably, at least25° C. below, most preferably at least 50° C. below, for example, atleast 100° C. below the temperature of the suspension in the reactorvessel. Typically, the heat exchange liquid is fed to the internal heatexchanger at a temperature of less than 100° C., preferably less than50° C. A suitable heat exchange liquid is water, a solution of aninorganic salt, molten inorganic salts, a high boiling point oil, aglycol or liquid sodium, preferably water.

[0029] Preferably, the internal heat exchanger comprises an array ofcooling tubes and/or cooling coils and/or cooling plates. Suitably, thearray may be divided into independently operated banks of cooling tubesand/or cooling coils and/or cooling plates (hereinafter “banks”).Preferably, the array comprises 5 to 500, more preferably 50 to 500,most preferably 100 to 500 such banks. Suitably, a bank comprises 5 to50 cooling tubes, or 5 to 20 cooling coils or 5 to 20 cooling plates.The amount of heat which may be removed from the system using the arraymay be adjusted by (a) independently adjusting the temperature of theheat exchange liquid which is fed to the banks and/or (b) increasing ordecreasing the number of banks to which the heat exchange fluid is fed.However, it is preferred to supply heat exchange fluid to all of thebanks of the array. Suitably, the temperature of the heat exchange fluidwhich is fed to at least some of the banks is at least 12° C. below,preferably at least 25° C. below, more preferably at least 50° C. below,for example, at least 100° C. below the temperature of the suspension inthe reactor vessel.

[0030] Where the heat exchanger comprises an array of cooling tubes, itis preferred that the cooling tubes are arranged substantially parallelto one another with the longitudinal axes of the cooling tubes alignedwith the longitudinal axis of the reactor vessel. Preferably, thecooling tubes have an outer diameter in the range 0.625 to 15 cm, morepreferably 1.25 to 7.5 cm, most preferably 2 to 5 cm, for example 2.5cm. Preferably, the cooling tubes may be finned so as to provide agreater heat transfer surface area within the reactor vessel. Suitably,the cooling tubes lie below the level of suspension in the reactorvessel and preferably extend through substantially the whole of theheight of the suspension in the reactor vessel, preferably through up to80% of the height of the suspension in the reactor, more preferablythrough up to 60% of the height of the suspension in the reactor vessel.Preferably, the cooling tubes are spaced from each other or from thewalls of the reactor by 5 to 60 cm, preferably, 7.5 to 25 cm, morepreferably 10 to 20 cm, for example, 12.5 cm. Suitably, cooling tubesare absent from the “blast” or “injection” zone(s) of the high shearmixing zone(s) i.e. the region of the reactor vessel into which the highshear mixing zone(s) discharges its contents. Where the high shearmixing zone(s) has a circular outlet, the “blast” or “injection” zonelies within a cylindrical region of the reactor vessel. The centre ofthe circular section of this cylindrical region is aligned with thecentre of the outlet of the high shear mixing zone. Suitably, thediameter of the circular section of the cylindrical region is at least 2times, preferably at least 3 times the diameter of the outlet of thehigh shear mixing zone.

[0031] Where the heat exchanger comprises an array of cooling coils,each cooling coil may be in the form of a helix with the coil wound asif along a cylinder (hereinafter “cylinder defined by the helix”). It ispreferred that the “blast” or “injection” zone of a high shear mixingzone lies within the cylinder defined by the helix. Suitably, thediameter of the cylinder defined by the helix is at least 2 times thediameter of the outlet of a high shear mixing zone, preferably at least3 times. Preferably, the tubing of the cooling coils has an outerdiameter of between 2.5 cm and 10 cm. Preferably, the cooling coils arefinned so as to provide a greater heat transfer surface area. Suitably,the cooling coils are spaced apart from each other or from the walls ofthe reactor vessel by 5 to 60 cm, preferably, 7.5 to 25 cm, morepreferably 10 to 20 cm, for example, 12.5 cm. Suitably, the coolingcoils lie below the level of suspension in the reactor vessel asdescribed above for the cooling tubes.

[0032] Where the heat exchanger comprises an array of cooling plates, itis preferred that cooling plates are concertinaed or corrugated so as toincrease the heat transfer area. Preferably, the cooling plates have abreadth of 2 to 10 cm. Preferably, the cooling plates have a depth(distance across the folds of the concertinaed plates or between thepeaks and troughs of the corrugated plates) of 10 to 50 cm. Preferably,the cooling plates are spaced apart from each other and from the wallsof the reactor vessel by at least 10 cm. Preferably, the longitudinalaxes of the cooling plates are aligned with the longitudinal axis of thereactor vessel. Suitably, where the cooling plates are arranged withtheir longitudinal axes aligned with the vertical axis of the reactorvessel, the cooling plates have a length of 60 to 80% of the height ofthe suspension in the reactor vessel.

[0033] Further cooling may also be provided to the system by introducinga liquid coolant to the reactor vessel and/or the high shear mixingzone(s) and/or to the external conduit. Preferably, the introduction ofthe liquid coolant removes at least 5%, preferably at least 10% of theexothermic heat of reaction from the system. The liquid coolant may beany liquid which is compatible with a Fischer-Tropsch synthesisreaction. Preferably, the liquid coolant which is to be introduced intothe system is at a temperature which is substantially below thetemperature of the suspension in the reactor vessel. Preferably, theliquid coolant is at a temperature which is at least 25° C. below,preferably at least 50° C. below, more preferably at least 100° C. belowthe temperature of the suspension in the reactor vessel. Suitably, theliquid coolant is cooled (e.g. using refrigeration techniques) beforebeing introduced into the system. Preferably, the liquid coolant iscooled to a temperature below 15° C., more preferably, below 10° C.

[0034] Preferably, the liquid coolant is a solvent which is capable ofvaporizing under the process conditions (i.e. at an elevated temperatureand pressure). Such a liquid coolant is hereinafter referred to as“vaporizable liquid coolant”). Without wishing to be bound by any theoryit is believed that the latent heat of vaporization of the vaporizableliquid coolant removes at least some of the exothermic heat of reactionfrom the system.

[0035] Suitably, the vaporizable liquid coolant has a boiling point, atstandard pressure, in the range of from 30 to 280° C., preferably from30 to 100° C. Preferably, the vaporizable liquid coolant is selectedfrom the group consisting of aliphatic hydrocarbons having from 5 to 10carbon atoms, alcohols (preferably, alcohols having from 1 to 4 carbonatoms, in particular, methanol and ethanol or a glycol), ethers (forexample, dimethyl ether) tetrahydrofuran, and water. In order tosimplify the process, it is preferred that the vaporizable liquidcoolant is selected from the group consisting of water (a by-product ofthe Fischer-Tropsch synthesis reaction) and low boiling liquid higherhydrocarbons, such as higher hydrocarbon having from 5 to 10 carbonatoms, in particular, pentanes, hexanes, or hexenes.

[0036] The high shear mixing zone(s) may be part of the system inside oroutside the reactor vessel, for example, the high shear mixing zone(s)may project through the walls of the reactor vessel such that the highshear mixing zone(s) discharges its contents into the reactor vessel.Preferably, the reactor system comprises a plurality of high shearmixing zones. Preferably the reactor system comprises 2 to 500 highshear mixing zones, more preferably 10 to 400 high shear mixing zones,most preferably, 20 to 300, for example, 25 to 150 high shear mixingzones. The high shear mixing zones may discharge into a single reactorvessel which has an advantage of significantly reducing the size of acommercial plant. However, it is also envisaged that the high shearmixing zones may discharge into 2 or 3 reactor vessels which areconnected in series. Where the reactor system comprises 2 or 3 reactorvessels, the high shear mixing zones may be divided substantiallyequally between the reactor vessels in the series. However, it is alsoenvisaged that high shear mixing zones may be unequally distributedbetween the reactor vessels in the series. Where the reactor systemcomprises a plurality of high shear mixing zones, the cooling tubesand/or cooling coils and/or cooling plates of the internal heatexchanger are substantially absent from the “blast” or “injection” zonesof each of the high shear mixing zones.

[0037] The high shear mixing zone(s) may comprise any device suitablefor intensive mixing or dispersing of a gaseous stream in a suspensionof solids in a liquid medium, for example, a rotor-stator device, aninjector-mixing nozzle or a high shear pumping means such as a propelleror paddle having high shear blades.

[0038] Preferably, the high shear mixing zone(s) is an injector mixingnozzle(s). Preferably the injector mixing nozzle(s) is at or near thetop of the reactor vessel and projects downwardly into the reactorvessel i.e. is a downshot nozzle. However, it is envisaged that theinjector mixing nozzle(s) may be at or near the bottom of the reactorvessel and projects upwardly into the reactor vessel i.e. is an upshotnozzle. The nozzle may also be angled, preferably at an angle of no morethan 25°, preferably no more than 10°, more preferably, no more than 5°relative to the longitudinal axis of the reactor vessel. Suitably, aplurality of injector mixing nozzles are spaced apart in the reactorvessel, preferably at or near the top or bottom of the reactor vessel sothat there is substantially no overlap between the blast zones of thenozzles. It is also envisaged that the reactor vessel may be providedwith additional nozzles (injector mixing nozzles, liquid only nozzles orgas only nozzles) which may be used to create turbulence in anyquiescent regions of the reactor vessel thereby avoiding sedimentationof the particulate catalyst.

[0039] The outlet of the nozzle(s) may be tapered outwardly so that thespray which exits the nozzle (suspension having gas bubbles and/orirregularly shaped gas voids dispersed therein) diverges outwardly,preferably, at an angle of less than 60°, more preferably at an angle ofless than 40°, most preferably at an angle of less than 30° relative tothe initial direction of discharge of the spray, for example, divergesoutwardly relative to the longitudinal axis of the reactor vessel.

[0040] The injector-mixing nozzle(s) can advantageously be executed as aventuri tube (c.f. “Chemical Engineers' Handbook” by J. H. Perry, 3^(rd)edition (1953), p.1285, FIG. 61), preferably an injector-mixer (c.f.“Chemical Engineers' Handbook” by J H Perry, 3^(rd) edition (1953), p1203, FIG. 2 and “Chemical Engineers' Handbook” by R H Perry and C HChilton 5^(th) edition (1973) p 6-15, FIG. 6-31) or most preferably asliquid-jet ejector (c.f. “Unit Operations” by G G Brown et al , 4^(th)edition (1953), p.194, FIG. 210). Where the injector-mixing nozzle(s) isa venturi tube, the constriction within the tube generally has adiameter of from 2.5 to 50 cm, preferably 5 to 25 cm. Alternatively, theinjector-mixing nozzle(s) may be executed as a “gas blast” or “gasassist” nozzle where gas expansion is used to drive the nozzle (c.f.“Atomisation and Sprays” by Arthur H Lefebvre, Hemisphere PublishingCorporation, 1989). Where the injector-mixing nozzle(s) is executed as a“gas blast” or “gas assist” nozzle, the suspension of catalyst is fed tothe nozzle at a sufficiently high pressure to allow the suspension topass through the nozzle while the gaseous stream comprising synthesisgas is fed to the nozzle at a sufficiently high pressure to achieve highshear mixing within the nozzle.

[0041] The high shear mixing zone(s) may also comprise an open-endedconduit having a venturi plate located therein. Preferably, the venturiplate is located close to the open end of the conduit, for example,within 0.5 metres of the open end of the conduit. Suspension is fed downthe conduit to the venturi plate at a sufficiently high pressure to passthrough apertures in the plate while a gaseous stream comprisingsynthesis gas is drawn into the conduit through at least one opening,preferably 2 to 50 openings, in the wall of the conduit. Preferably, theopening(s) is located immediately downstream of the venturi plate.Suitably, the venturi plate has between 2 to 50 apertures. Preferably,the apertures are circular having diameters in the range of 1 mm to 100mm. Suspension having gas bubbles and/or irregularly shaped gas voidsdispersed therein is then discharged into the reactor vessel though theopen end of the conduit. Suitably, the open end of the conduit istapered outwardly, preferably at an angle of 5 to 30°, preferably 10 to25° relative to the longitudinal axis of the conduit. Conveniently, theexternal conduit may recycle the suspension recycle stream to theopen-ended conduit.

[0042] Where the high shear mixing zone(s) is executed as a venturinozzle(s) (either as a venturi tube or as a venturi plate), the pressuredrop of the suspension over the venturi nozzle(s) is typically in therange of from 1 to 40 bar, preferably 2 to 15 bar, more preferably 3 to7 bar, most preferably 3 to 4 bar. Preferably, the ratio of the volumeof gas (Q_(g)) to the volume of liquid (Q_(l)) passing through theventuri nozzle(s) is in the range 0.5:1 to 10:1, more preferably 1:1 to5:1, most preferably 1:1 to 2.5:1, for example, 1:1 to 1.5:1 (where theratio of the volume of gas (Q_(g)) to the volume of liquid (Q_(l)) isdetermined at the desired reaction temperature and pressure).

[0043] Where the high shear mixing zone(s) is executed as a gas blast orgas assist nozzle(s), the pressure drop of gas over the nozzle(s) ispreferably in the range 3 to 100 bar and the pressure drop of suspensionover the nozzle(s) is preferably in the range of from 1 to 40 bar,preferably 4 to 15, most preferably 4 to 7. Preferably, the ratio of thevolume of gas (Q_(g)) to the volume of liquid (Q_(l)) passing throughthe gas blast or gas assist nozzle(s) is in the range 0.5:1 to 50:1,preferably 1:1 to 10:1 (where the ratio of the volume of gas (Q_(g)) tothe volume of liquid (Q_(l)) is determined at the desired reactiontemperature and pressure).

[0044] The high shear mixing zone(s) may also comprise an open-endedconduit having a high shear pumping means positioned therein, forexample a paddle or propeller having high shear blades. Suitably, thehigh shear pumping means is located close to the open end of the conduitfor example, within 1 metre, preferably within 0.5 metres of the openend of the conduit. A gaseous stream comprising synthesis gas isinjected into the open-ended conduit either immediately upstream ordownstream of the high shear pumping means, preferably, immediatelyupstream of the high shear pumping means. By immediately upstream ismeant that the gaseous stream is injected into the open-ended conduitless than 0.25 metres before the high shear pumping means. The gaseousstream may be injected into the open-ended conduit by means of asparger. Without wishing to be bound by any theory, the injected gaseousstream is broken down into gas bubbles and/or irregularly shaped gasvoids (hereinafter “gas voids) by the fluid shear imparted to thesuspension by the high shear pumping means and the resulting gas bubblesand/or gas voids become entrained in the suspension. The resultingsuspension containing entrained gas bubbles and/or gas voids is thendischarged into the reactor vessel through the open end of the conduit.Suitably, the open end of the conduit is tapered outwardly, preferablyat an angle of 5 to 25°, preferably 10 to 20° relative to thelongitudinal axis of the conduit. Conveniently, the external conduit mayrecycle the suspension recycle stream to the open-ended conduit.

[0045] Suitably, the volume of suspension present in the high shearmixing zone(s) is substantially less than the total volume of suspensionpresent in the reactor system, for example, less than 20%, preferablyless than 10% of the total volume of suspension present in the reactorsystem.

[0046] Preferably, the fluid shear imparted to the suspension in thehigh shear mixing zone(s) breaks down at least a portion of the gaseousreactant stream into gas bubbles having diameters in the range of from 1μm to 10 mm, preferably from 30 μm to 3000 μm, more preferably from 30μm to 300 μm which then become entrained in the suspension.

[0047] Without wishing to be bound by any theory, it is believed thatthe irregularly shaped gas voids are transient in that they arecoalescing and fragmenting on a time scale of up to 500 ms, for example,over a 10 to 500 ms time scale. The irregularly shaped gas voids have awide size distribution with smaller gas voids having an average diameterof 1 to 2 mm and larger gas voids having an average diameter of 10 to 15mm.

[0048] Suitably, kinetic energy is dissipated to the suspension presentin the high shear mixing zone(s) at a rate of at least 0.5 kW/m³relative to the total volume of suspension present in the system.Preferably, the kinetic energy dissipation rate in the high shear mixingzone is in the range of from 0.5 to 25 kW/m³, relative to the totalvolume of suspension present in the system, more preferably from 0.5 to10 kW/m³, most preferably from 0.5 to 5 kW/m³, and in particular, from0.5 to 2.5 kW/m³.

[0049] In a preferred embodiment the process is carried out using atleast one injector mixing nozzle, preferably a plurality of injectormixing nozzles. Very good mixing can be achieved when theinjector-mixing nozzle(s) is situated at or near the top of the reactorvessel and the suspension is removed from the reactor vessel at or nearits bottom. Therefore the reactor vessel is preferably provided at ornear its top with at least one, preferably a plurality ofinjector-mixing nozzles and the suspension recycle stream is preferablywithdrawn from at or near the bottom of the reactor vessel. Preferably,the suspension recycle stream is, at least in part recycled via anexternal conduit, having a slurry pump positioned therein, to the top ofthe injector-mixing nozzle(s) through which it is then injected into thetop of the reactor vessel. The gaseous stream comprising synthesis gasis introduced through one or more openings in the side wall of theinjector-mixing nozzle(s). An internal heat exchanger comprising anarray of cooling tubes and/or cooling coils and/or cooling plates islocated in the reactor vessel in regions which are outside of the blastzone(s) of the nozzle(s) so that the spray from the nozzle(s) does notimpinge on the array. Without wishing to be bound by any theory it isbelieved that if the array extends into the blast zone(s) of thenozzle(s) that this interferes with the fluid dynamics in the reactorvessel and may also result in coalescence of gas bubbles and erosion ofthe cooling tubes and/or cooling coils, and/or cooling plates of thearray. Preferably, an external heat-exchanger is positioned on theexternal conduit to remove at least a portion of the exothermic heat ofreaction from the system.

[0050] As discussed above, a gas cap (containing unconverted synthesisgas, gaseous higher hydrocarbons, vaporized higher hydrocarbons,vaporized water, any vaporized liquid coolant and any inert gases) maybe present in the top of reactor vessel above the level of thesuspension. Suitably, the volume of the gas cap is not more than 40%,preferably not more than 30% of the volume of the reactor vessel. Thehigh shear mixing zone(s) may discharge into the reactor vessel eitherabove or below the level of suspension in the reactor vessel. Anadvantage of the high shear mixing zone(s) discharging below the levelof suspension is that this improves the contact between the synthesisgas and the suspension in the reactor vessel.

[0051] A gaseous recycle stream may be withdrawn from the headspace ofthe reactor vessel and may be recycled to the high shear mixing zone(s).The gaseous recycle stream is preferably cooled before being recycled tothe high shear mixing zone(s), for example, by passing the gaseousrecycle stream through a heat exchanger. The gaseous recycle stream maybe cooled to a temperature at which a two phase mixture of gas(comprising unconverted synthesis gas, methane by-product, gaseoushigher hydrocarbons and any inert gases, for example, nitrogen) andcondensed liquid (water by-product, low boiling liquid higherhydrocarbons and any liquid coolant) is formed. The condensed liquid maybe separated from the gaseous recycle stream, for example, using asuitable gas-liquid separation means (e.g. a hydrocyclone, demister,gravity separator) and at least a portion of the condensed liquid may berecycled to the reactor vessel or the high shear mixing zone(s), forexample, with fresh liquid coolant. Preferably, excess water by-productis removed from the separated condensed liquids using a suitableseparation means (e.g. a decanter), before recycling the condensedliquids to the reactor vessel or high shear mixing zone(s). It isenvisaged that the heat exchanger and gas-liquid separation means may becombined within a single unit in order to simplify recycling of thegaseous stream.

[0052] Fresh synthesis gas may be fed to the gaseous recycle stream,either upstream or downstream of the external exchanger. Where thesynthesis gas has not been pre-cooled, the synthesis gas may be fed tothe gaseous recycle stream upstream of the heat exchanger. Preferably,the gaseous recycle stream is recycled to the high shear mixing zone(s)via a blower or compressor located downstream of the external heatexchanger.

[0053] Preferably, a purge stream is taken from the gaseous recyclestream to prevent the accumulation of gaseous by-products, for example,methane or carbon dioxide, or any inert gases, for example, nitrogen inthe system. If desired, any gaseous intermediate products may beseparated from the purge stream. Preferably, such gaseous intermediateproducts are recycled to the reactor vessel where they may be convertedto higher hydrocarbons.

[0054] In a further aspect of the present invention there is provided areactor system for converting synthesis gas to higher hydrocarbons inthe presence of a suspension comprising a particulate Fischer-Tropschcatalyst suspended in a liquid medium, said reactor system comprising(i) a reactor vessel, (ii) at least one high shear mixing zone having anoutlet within the reactor vessel, (iii) an external conduit having afirst end in fluid communication with an outlet of the reactor vessel,said outlet being located at or near the bottom of the reactor vessel,and a second end in fluid communication with the high shear mixingzone(s), (iv) a first heat exchanger positioned within the reactorvessel and (v) a second heat exchanger positioned on the externalconduit characterized in that the heat exchange surfaces of the firstheat exchanger are located within regions of the reactor which areoutside of the blast zone(s) of the high shear mixing zone(s).

[0055] Preferred features of the reactor system are as described abovein relation to the process of the present invention.

[0056] Preferably, the ratio of hydrogen to carbon monoxide in thesynthesis gas used in the process of the present invention is in therange of from 20:1 to 0.1:1, especially 5:1 to 1:1 by volume, typically2:1 by volume based on the total amount of hydrogen and carbon monoxideintroduced to the system.

[0057] The synthesis gas may be prepared using any of the processesknown in the art including partial oxidation of hydrocarbons, steamreforming, gas heated reforming, microchannel reforming (as describedin, for example, U.S. Pat. No. 6,284,217 which is herein incorporated byreference), plasma reforming, autothermal reforming, and any combinationthereof. A discussion of these synthesis gas production technologies isprovided in “Hydrocarbon Processing” V78, N.4, 87-90, 92-93 (April 1999)and “Petrole et Techniques”, N. 415, 86-93 (July-August 1998). It isalso envisaged that the synthesis gas may be obtained by catalyticpartial oxidation of hydrocarbons in a microstructured reactor asexemplified in “IMRET 3: Proceedings of the Third InternationalConference on Microreaction Technology”, Editor W Ehrfeld, SpringerVerlag, 1999, pages 187-196. Alternatively, the synthesis gas may beobtained by short contact time catalytic partial oxidation ofhydrocarbonaceous feedstocks as described in EP 0303438. Preferably, thesynthesis gas is obtained via a “Compact Reformer” process as describedin “Hydrocarbon Engineering”, 2000, 5, (5), 67-69; “HydrocarbonProcessing”, 79/9, 34 (September 2000); “Today's Refinery”, 15/8, 9(August 2000); WO 99/02254; and WO 200023689.

[0058] Preferably, the higher hydrocarbons produced in the process ofthe present invention comprise a mixture of hydrocarbons having a chainlength of greater than 2 carbon atoms, typically greater than 5 carbonatoms. Suitably, the higher hydrocarbons comprise a mixture ofhydrocarbons having chain lengths of from 5 to about 90 carbon atoms.Preferably, a major amount, for example, greater than 60% by weight, ofthe higher hydrocarbons have chain lengths of from 5 to 30 carbon atoms.Suitably, the liquid medium comprises one or more higher hydrocarbonswhich are liquid under the process conditions.

[0059] The catalyst which may be employed in the process of the presentinvention is any catalyst known to be active in Fischer-Tropschsynthesis. For example, Group VIII metals whether supported orunsupported are known Fischer-Tropsch catalysts. Of these iron, cobaltand ruthenium are preferred, particularly iron and cobalt, mostparticularly cobalt.

[0060] A preferred catalyst is supported on a carbon based support, forexample, graphite or an inorganic oxide support, preferably an inorganicrefractory oxide support. Preferred supports include silica, alumina,silica-alumina, the Group IVB oxides, titania (primarily in the rutileform) and most preferably zinc oxide. The supports generally have asurface area of less than about 100 m²/g, preferably less than 50 m²/g,more preferably less than 25 m²/g, for example, about 5 m²/g.

[0061] The catalytic metal is present in catalytically active amountsusually about 1-100 wt %, the upper limit being attained in the case ofunsupported metal based catalysts, preferably 2-40 wt %. Promoters maybe added to the catalyst and are well known in the Fischer-Tropschcatalyst art. Promoters can include ruthenium, platinum or palladium(when not the primary catalyst metal), aluminium, rhenium, hafnium,cerium, lanthanum and zirconium, and are usually present in amounts lessthan the primary catalytic metal (except for ruthenium which may bepresent in coequal amounts), but the promoter:metal ratio should be atleast 1:10. Preferred promoters are rhenium and hafnium.

[0062] The catalyst may have a mean particle size in the range 5 to 500microns, preferably 10 to 250 microns, for example, in the range 10 to30 microns.

[0063] Preferably, the suspension of catalyst discharged into thereactor vessel comprises less than 50% wt of catalyst particles, morepreferably 10 to 40% wt of catalyst particles, most preferably 10 to 30%wt of catalyst particles.

[0064] The process of the invention is preferably carried out at atemperature of 180-380° C., more preferably 180-280° C., mostpreferably, 190-240° C.

[0065] The process of the invention is preferably carried out at apressure of 5-50 bar, more preferably 15-35 bar, generally 20-30 bar.

[0066] The process of the present invention can be operated in batch orcontinuous mode, the latter being preferred.

[0067] Suitably, the gas hourly space velocity (GHSV) for a continuousprocess is in the range 100 to 40000 h⁻¹, more preferably 1000 to 30000h¹, most preferably 2000 to 15000 h⁻¹, for example 4000 to 10000 h⁻¹ atnormal temperature and pressure (NTP) based on the feed volume ofsynthesis gas at NTP.

[0068] In a continuous process, in order to achieve a sufficiently highproductivity, the suspension should be present in the reactor vessel fora certain period of time. It has been found that the average residencetime of the liquid phase (i.e. the liquid component of the suspension)in the reactor vessel is advantageously in the range from 15 minutes to50 hours, preferably 1 to 30 hours. Where two reactor vessels areoperated in series it is preferred that the average residence time ofthe liquid phase in the first reactor vessel is in the range of 15minutes to 50 hours, preferably 1 hour to 30 hours and the averageresidence time of the liquid phase in the second reactor vessel is inthe range of 30 minutes to 30 hours.

[0069] Suitably, the gas residence time in the high shear mixing zone(s)(for example, the injector-mixing nozzle(s) is in the range 20milliseconds to 2 seconds, preferably 50 to 250 milliseconds.

[0070] Suitably, the gas residence time in the reactor vessel is in therange 10 to 240 seconds, preferably 20 to 90 seconds.

[0071] Suitably, the gas residence time in the external conduit is inthe range 10 to 180 seconds, preferably 25 to 60 seconds.

[0072] In a continuous process, product suspension is continuouslyremoved from the system, preferably from the external conduit and ispassed to a liquid-solid separation means, where liquid medium andliquid higher hydrocarbons are separated from the catalyst. Preferably,entrained gases are separated from the product suspension either withinthe liquid-solid separation means or prior to introducing the productsuspension to the liquid-solid separation mean, for example in theexternal heat exchanger. Examples of suitable liquid-solid separationmeans include hydrocyclones, filters, gravity separators, T-pieceseparators and magnetic separators. Alternatively, the liquid medium andliquid higher hydrocarbons may be separated from the catalyst bydistillation. Preferably, there are two or more product withdrawal lines(for two or more product side streams), each line leading to a dedicatedliquid-solid separation means. This ensures continuous operation of theprocess by allowing one or more of the liquid-solid separation means tobe taken off-line for cleaning. The separated liquids are then passed toa product purification stage where water by-product and liquid mediumare removed from the liquid higher hydrocarbons. The purification stagemay be simplified by using one or more of the liquid higher hydrocarbonsproduced in the process of the present invention as the liquid medium inwhich case there is no requirement to separate the liquid medium fromthe liquid higher hydrocarbons. The catalyst may be recycled as aconcentrated slurry to the reactor vessel. Fresh catalyst may be addedeither to the recycled slurry or directly into the reactor vessel.

[0073] The liquid higher hydrocarbons from the purification stage may befed to a hydrocracking stage, for example, a catalytic hydrocrackingstage which employs a catalyst comprising a metal selected from thegroup consisting of cobalt, molybdenum, nickel and tungsten supported ona support material such as alumina, silica-alumina or a zeolite.Preferably, the catalyst comprises cobalt/molybdenum ornickel/molybdenum supported on alumina or silica-alumina. Suitablehydrocracking catalysts include catalysts supplied by Akzo Nobel,Criterion, Chevron, or UOP.

[0074] The invention will now be illustrated with the aid of FIGS. 1 to3.

[0075]FIG. 1 illustrates a reactor system according to the presentinvention comprising a reactor vessel (1) and a plurality of venturiinjector-mixing nozzles (2). A gas cap (3) is present in the upper partof the reactor vessel (1), the lower part of which contains a suspension(4) of particulate catalyst suspended in the liquid higher hydrocarbons.A dotted line (5) denotes the upper level of the suspension (4) in thereactor vessel (1). Suspension (4) is recycled to the venturiinjector-mixing nozzles (2) via a line (6) and dedicated branch lines(7). Through one or more openings in the side walls of the venturiinjector-mixing nozzles (2) a gaseous phase comprising synthesis gas isdrawn into the nozzles (1) from the gas cap (3). Fresh synthesis gas isintroduced into the gas cap (3) via a line (8).

[0076] The suspension is cooled within the reactor vessel (1) by meansof a plurality of cooling tubes (9) positioned within the reactor vessel(1) below the upper level of the suspension (5) and which are locatedoutside of the blast zones of the nozzles (2).

[0077] Via a lower outlet opening of the injector-mixing nozzles (2) thesuspension having synthesis gas entrained therein is discharged into thereactor vessel (1) below the level (5) of the suspension (4).Unconverted gaseous reactants then separate into the gas cap (3).

[0078] Suspension (4) is withdrawn from the bottom of the vessel (1) andat least a portion of the suspension is recycled to the injector-mixingnozzles (2) by means of pump (10) and the line (6). The suspensionpassing through line (6) is cooled by means of an external heatexchanger (11).

[0079] Via a line (12) a portion of the suspension (4) is withdrawn fromthe system and is passed to a liquid-solid separation stage (not shown).

[0080]FIG. 2 illustrates a schematic plan view of the reactor vessel (1)showing the arrangement of the nozzles (2) and the cooling tubes (9).

[0081]FIGS. 3A and 3B illustrate a cooling tube (9) and a finned coolingtube (13) while FIG. 3C illustrates a cooling plate (14).

EXAMPLE 1

[0082] It was calculated that for a 30,000 barrel a day commercialplant, between 500 and 550 MegaWatts of heat must be removed from thesystem to maintain the temperature of the suspension within the reactorvessel at the desired reaction temperature, ideally, isothermal. Theamount of heat generated will depend on the type of catalyst and theproduct distribution.

[0083] Table 1 shows the relationship between the diameter of theconstriction of a venturi nozzle and the number of venturi nozzles whichwould be required for a 30,000 barrel per day plant (where the lowervalue for the number of nozzles corresponds to a Q_(g):Q_(l) ratio of1:1.5 and the higher value for the number of nozzles corresponds to aQ_(g):Q_(l) ratio of 2.5:1). TABLE 1 Number of venturi nozzles for a30,0000 bbl/d plant Diameter of restriction in Number of VenturiPressure in Reactor Nozzle(inch) Nozzles Bbl/d/nozzle Vessel (bara) 2345-207  87-145 20 3 153-93  195-326 20 4 86-52 348-579 20 4 65-39463-772 30 5 56-33 539-898 20 5 42-25  718-1197 30 6 40-24  752-1253 206 30-18 1002-1670 30

[0084] Table 2 illustrates the % reduction in the rate at whichsuspension is required to be fed to an external heat exchange (tomaintain the desired reaction temperature in the reactor vessel) withincreasing cooling of the suspension in the external heat exchanger. Inthe base case, the heat exchanger is operated with the suspension beingcooled to a temperature 12° C. below that of the suspension in thereactor vessel. It was found that in order to maintain substantiallyisothermal conditions in the reactor vessel, the suspension must bepassed to the heat exchanger at a rate of 51,000 m³ of suspension perhour. TABLE 2 Percentage Reduction in suspension recycle % reduction insuspension recycle ΔT (° C.) — 12 20 15 40 20 52 25 60 30

[0085] However, the desired reduction in the rate of recycle to theexternal heat exchanger cannot be achieved using only an external heatexchanger owing to the risk of quenching of the Fischer-Tropschsynthesis reaction and/or deactivation of the catalyst in the externalheat exchanger unit. Furthermore, without the presence of the internalheat exchanger unit it would not be possible to control adequately thetemperature of the suspension in the reaction vessel. It is thereforenecessary to remove at least a portion of the exothermic heat ofreaction from the system by means of an internal heat exchangerpositioned within the suspension in the reactor vessel. TABLE 3 Heatremoval from a 30,000 bbl/day plant % Heat % Heat removed by removed bymake-up vaporizing % Heat removed by % Heat removed by feeds (freshliquid External Heat Internal Heat synthesis gas) coolant ExchangerExchanger 5 0 95 0 51,000 m³/h recycle 5 5 50 40 27,000 m³/h recycle 105 50 30 27,000 m³/h recycle 10 10 30 50 15,500 m³/h recycle

[0086] Table 3 illustrates how heat may be removed from a 30,000 bbl/dayFischer Tropsch plant via an internal heat exchanger, an external heatexchangers and by injecting a stream comprising a vaporizable liquidcoolant. The total heat production of the plant is 505 MegaWatts.

EXAMPLE 2

[0087] A suspension of aluminium oxide catalyst (15% w/w) in 800 litresof tetradecene was charged to a reactor system comprising a tank reactorvessel (straight length 4500 mm, diameter 420 mm) and an externalconduit. A gas liquid separator, a first and a second three-phasecentrifugal pump were positioned on the external conduit (the firstcentrifugal pump (P1080) located upstream and the second centrifugalpump (P1280) located downstream of the gas liquid separator). A 24 mmventuri nozzle was located in the upper region of the tank reactorvessel. Tetradecene was used as a mimic for the Fischer Tropsch wax,having similar physical properties at 30° C. as Fischer Tropsch wax at atemperature of between 200-250° C. Nitrogen was used as the gas feed tothe venturi nozzle and suspension having gas bubbles and irregularlyshaped gas voids dispersed therein was discharged from the nozzle belowthe level of suspension in the tank reactor vessel. The reactor systemwas pressurised from 0 to 30 bar (depending on the experimentalconditions) using nitrogen and the suspension was pumped around theexternal conduit using the three-phase centrifugal pumps. The reactorsystem was then allowed to reach steady state.

[0088] A series of measurements was taken around the loop in the absenceof any internal reactor cooling tubes, as detailed in Table 4 below. Aseries of cooling tubes were then introduced into the tank reactorvessel. The cooling tubes had an outer diameter of 25.4 mm. The coolingtubes were arranged in the tank reactor vessel such that there were 6equi-spaced tubes located on a 200 mm PCD (PCD=pitch circle diameter)and 12 equi-spaced tubes on a 340 mm PCD. The series of measurements wasthen repeated with the cooling tubes in place. The results are shown inTable 5 below. TABLE 4 No cooling tubes System pressure 20 bargnitrogen; Suspension recirculation rate 30 tonnes/hr gas/liquid ratio nogas 1:1 P1080% 87 87 P1080 rpm 1479 1479 Pressure drop across P1080(bar) 3.01 3.01 P1080 head (m) 35.59 35.59 P1280% 84 88 Pressure dropacross P1280 bar 3.01 3.29 P1280 rpm 1428 1496 P 1280 head (m) 35.5938.9 Pressure in gas liquid separator (bar) 19.93 20.01 Pressure inexternal conduit downstream of P1280 22.94 23.3 (bar) Pressure drop inventuri nozzle suction chamber (bar) 1.03 0.74 FV1092% (position of flowvalve on gas recycle line 0 32 from the reactor headspace to the venturinozzle) Flow rate of gaseous recycle stream from reactor 0 833 headspaceto the venturi nozzle (kg/hr) Flow rate of gaseous recycle stream fromthe gas 0 0 liquid separator to the venturi nozzle (kg/hr) Pressure inthe tank reactor vessel (bar) 19.94 20.03 Pressure drop of suspensionacross venturi nozzle 2.55 2.8 (bar) Suspension circulation rate(tonnes/hr) 30 31

[0089] TABLE 5 Cooling tubes System pressure 20 barg nitrogen;Suspension recirculation rate 30 tonnes/hr gas/liquid ratio No gas 1:1P1080% 90 90 P1080 rpm 1530 1530 Pressure drop across P1080 bar 3.143.14 P1080 head (m) 37.12 37.12 P1280% 100 100 Pressure drop acrossP1280 bar 2.8 3.11 P1280 rpm 1700 1700 P1280 head (m) 33.1 36.77Pressure in gas liquid separator (bar) 19.97 20.19 Pressure in externalconduit downstream of P1280 22.77 23.3 (bar) Pressure drop over venturinozzle suction chamber 0.87 0.51 (bar) FV1092% (Position of flow valveon gaseous recycle 0 32 line from reactor head space to the venturinozzle) Flow rate of gaseous recycle stream from reactor 0 735 headspaceto venturi nozzle (kg/hr) Flow rate of gaseous recycle stream from gasliquid 0 0 separator to venturi nozzle (kg/hr) Pressure in reactorvessel (bar) 20 20.33 Pressure drop of suspension across nozzle (bar)2.02 2.39 Suspension circulation rate (tonnes/hr) 27 29

1. A process for the conversion of synthesis gas to higher hydrocarbonsby contacting a gaseous stream comprising synthesis gas, at an elevatedtemperature and pressure, with a suspension comprising a particulateFischer-Tropsch catalyst suspended in a liquid medium, in a systemcomprising at least one high shear mixing zone and a reactor vesselwherein the process comprises: (a) passing the suspension and thegaseous stream through the high shear mixing zone(s) wherein the gaseousstream is broken down into gas bubbles and/or irregularly shaped gasvoids; (b) discharging suspension having gas bubbles and/or irregularlyshaped gas voids dispersed therein from the high shear mixing zone(s)into the reactor vessel; (c) maintaining the temperature of thesuspension discharged into the reactor vessel at the desired reactiontemperature by means of an internal heat exchanger positioned within thesuspension in the reactor vessel characterized in that at least 5% ofthe exothermic heat of reaction is removed from the system by means ofthe internal heat exchanger.
 2. A process as claimed in claim 1 whereinat least 10%, preferably at least 20% of the exothermic heat of reactionis removed from the system by means of the internal heat exchanger.
 3. Aprocess as claimed in claim 2 wherein between 20 to 50%, preferably 30to 50% of the exothermic heat of reaction is removed from the system bymeans of the internal heat exchanger.
 4. A process as claimed in any oneof the preceding claims wherein suspension is withdrawn from the reactorvessel and is at least in part recycled to the high shear mixing zone(s)after being cooled by means of an external heat exchanger.
 5. A processas claimed in claim 4 wherein between 20 to 55%, preferably 30 to 50% ofthe exothermic heat of reaction is removed from the system in theexternal heat exchanger.
 6. A process as claimed in claims 4 or 5wherein the suspension recycle stream is cooled by means of the externalheat exchanger to a temperature not more than 30° C. below, preferably,not more 12° C. below, the temperature of the suspension in the reactorvessel.
 7. A process as claimed in any one of claims 4 to 6 wherein thesuspension is withdrawn from the reactor vessel and is recycled to thehigh shear mixing zone(s) by means of an external conduit having a firstend in communication with an outlet for the suspension in the reactorvessel and a second end in communication with the high shear mixingzone(s) and the external heat exchanger is positioned on the externalconduit.
 8. A process as claimed in claim 7 wherein the ratio of thevolume of the external conduit (excluding the volume of the externalheat exchanger) to the volume of the reactor vessel is in the range of0.005:1 to 0.2:1.
 9. A process as claimed in claims 7 or 8 wherein thesuspension is recycled through the external conduit at a rate of between(n×10,000) m³/h to (n×50,000) m³/h for a (n×30,000) barrel/daycommercial plant where n is a number in the range 0.25 to
 10. 10. Aprocess as claimed in any one of the preceding claims wherein thereactor vessel approximates to a continuous stirred tank reactor (CSTR)having a Peclet number of less than 3 where the Peclet (Pe) number isdefined by the equation: Pe=U _(g) H/δ where U_(g) is the gas velocity(ms⁻¹), H is the height of the suspension in the reactor vessel (m), andδ is the dispersion coefficient (m²s⁻¹).
 11. A process as claimed in anyone of the preceding claims wherein a heat exchange liquid is fed to theinternal heat exchanger at a temperature which is at least 25° C. below,more preferably at least 50° C. below, for example, at least 100° C.below the temperature of the suspension in the reactor vessel.
 12. Aprocess as claimed in claim 11 wherein the heat exchange liquid fed tothe internal heat exchanger is selected from the group consisting ofwater, a solution of an inorganic salt, molten inorganic salts, a highboiling point oil and liquid sodium.
 13. A process as claimed in any oneof the preceding claims wherein the internal heat exchanger comprises anarray of cooling tubes and/or cooling coils and/or cooling plates.
 14. Aprocess as claimed in claim 13 wherein the array is divided into 50 to500 independently operated banks of cooling tubes and/or cooling coilsand/or cooling plates.
 15. A process as claimed in claims 13 or 14wherein the cooling tubes of the array are arranged with theirlongitudinal axes aligned with the longitudinal axis of the reactorvessel.
 16. A process as claimed in any one of claims 13 to 15 whereinthe cooling tubes have an outer diameter in the range 0.625 to 15 cm,more preferably 1.25 to 7.5 cm, and most preferably 2 to 5 cm and arespaced apart from each other or from the walls of the reactor vessel by5 to 60 cm, preferably, 7.5 to 25 cm, more preferably 10 to 20 cm.
 17. Aprocess as claimed in any one of claims 13 to 16 wherein the coolingtubes are absent from the “blast” zone(s) of the high shear mixingzone(s).
 18. A process as claimed in claims 13 or 14 wherein eachcooling coil of the array is in the form of a helix with the coil woundas if along a cylinder.
 19. A process as claimed in claim 18 wherein thetubing of the cooling coils has an outer diameter of between 2.5 cm and10 cm and the cooling coils are spaced apart from each other or from thewalls of the reactor vessel by 5 to 60 cm, preferably, 7.5 to 25 cm,more preferably 10 to 20 cm.
 20. A process as claimed in claims 18 or 19wherein the “blast” zone of a high shear mixing zone is arranged so asto lie within the cylinder defined by the helix of the cooling coil andthe diameter of the cylinder defined by the helix is at least 2 times,preferably at least 3 times, the diameter of the outlet of a high shearmixing zone.
 21. A process as claimed in claims 13 or 14 wherein theheat exchanger comprises an array of concertinaed or corrugated coolingplates having a breadth of from 2 to 10 cm and a depth (distance acrossthe folds of the concertinaed plates or between the peaks and troughs ofthe corrugated plates) of from 10 to 50 cm and the cooling plates arespaced apart from each other and from the walls of the reactor vessel byat least 10 cm.
 22. A process as claimed in any one of claims 13 to 21wherein the cooling tubes, cooling coils or cooling plates are finned.23. A process as claimed in any one of claims 13 to 22 wherein thecooling tubes, cooling coils or cooling plates lie below the level ofsuspension in the reactor vessel and extend through up to 80% of theheight of the suspension in the reactor vessel.
 24. A process as claimedin any one of the preceding claims wherein at least 5% of the exothermicheat of reaction is removed from the system by introducing a vaporizableliquid coolant to the reactor vessel and/or the high shear mixingzone(s) at a temperature which is at least 25° C. below, preferably atleast 50° C. below, more preferably at least 100° C. below thetemperature of the suspension in the reactor vessel.
 25. A process asclaimed in claim 24 wherein the vaporizable liquid coolant is selectedfrom the group consisting of aliphatic hydrocarbons having from 5 to 10carbon atoms, alcohols (preferably, alcohols having from 1 to 4 carbonatoms, in particular, methanol and ethanol), ethers (for example,dimethyl ether) tetrahydrofuran, and water.
 26. A process as claimed inany one of the preceding claims wherein the reactor system comprises 10to 400 high shear mixing zones, preferably, 20 to 300 high shear mixingzones and the high shear mixing zones discharge into a single reactorvessel or into 2 or 3 reactor vessels connected in series.
 27. A processas claimed in any of the preceding claims wherein the high shear mixingzone(s) comprise (a) an injector-mixing nozzle(s) or (b) an open-endedconduit(s) having a high shear pumping means positioned therein and agas sparger located immediately upstream or downstream of the high shearpumping means.
 28. A process as claimed in claims 27 wherein theinjector-mixing nozzle(s) is executed as a venturi nozzle(s) or as gasblast nozzle(s).
 29. A process as claimed in claim 28 wherein thepressure drop of the suspension over the venturi nozzle(s) is in therange of from 1 to 40 bar, preferably 2 to 15 bar and the ratio of thevolume of gas (Q_(g)) to the volume of liquid (Q_(l)) passing throughthe venturi nozzle(s) is in the range 0.5:1 to 10:1, preferably 1:1 to5:1 measured at the conditions of elevated temperature and pressure. 30.A process as claimed in claim 28 wherein the pressure drop of gas overthe gas blast nozzle(s) is in the range 3 to 100 bar, the pressure dropof the suspension over the gas blast nozzle(s) is in the range of from 1to 40 bar, preferably 4 to 15 bar and the ratio of the volume of gas(Q_(g)) to volume of liquid (Q_(l)) passing through the gas blastnozzle(s) is in the range 0.5:1 to 50:1, preferably 1:1 to 10:1 measuredat the conditions of elevated temperature and pressure.
 31. A process asclaimed in any one of claims 28 to 30 wherein the injector mixingnozzle(s) are located at or near the top of the reactor vessel andproject downwardly into the reactor vessel.
 32. A process as claimed inclaim 31 wherein the injector mixing nozzle(s) are angled at an angle ofno more than 25°, preferably at an angle of no more than 10°, morepreferably, at an angle of no more than 5° relative to the longitudinalaxis of the reactor vessel.
 33. A process as claimed in any one ofclaims 28 to 32 wherein the outlet of the nozzle(s) are taperedoutwardly so that the spray which exits the nozzle (suspension havinggas bubbles and/or gas voids dispersed therein) diverges outwardly at anangle of less than 60°, preferably at an angle of less than 40°, mostpreferably at an angle of less than 30° relative to the initialdirection of discharge of the spray.
 34. A process as claimed in any oneof the preceding claims wherein the fluid shear imparted to thesuspension in the high shear mixing zone(s) breaks down at least aportion of the gaseous stream comprising synthesis gas into gas bubbleshaving diameters in the range of from 1 μm to 10 mm, preferably from 30μm to 3000 μm, more preferably from 30 μm to 300 μm which bubbles becomeentrained in the suspension.
 35. A process as claimed in any one of thepreceding claims wherein the irregularly shaped gas voids dispersed inthe suspension discharged into the reactor vessel either coalesce toform larger gas voids or fragment to form smaller gas voids with the gasvoids having an average duration of up to 500 ms.
 36. A process asclaimed in any one of the preceding claims wherein kinetic energy isdissipated to the suspension present in the high shear mixing zone(s) ata rate of from 0.5 to 25 kW/m³, relative to the total volume ofsuspension present in the system.
 37. A process as claimed in any one ofthe preceding claims wherein the process is operated in continuous modeand the average residence time of the liquid component of the suspensionin the reactor vessel is in the range from 15 minutes to 50 hours,preferably 1 to 30 hours.
 38. A process as claimed in any one of claims4 to 37 wherein up to 50% by volume of the hydrogen component of thesynthesis and/or up to 50% by volume of the carbon monoxide component ofthe synthesis gas is introduced into the suspension recycle stream.